In "cab-over" trucks, the engine of the truck is positioned beneath the cab and the cab is tiltable, usually about a horizontal axis located at the front of the cab, to gain access to the engine for service. The front pivot axis may be either fixed to the chassis of the truck, which is known in the art as a "hard mount", or, as is more common in Europe, may be attached to the chassis by shock and vibration isolation mounts. However, regardless of whether isolation mounts are used to affix the front pivot axis to the chassis or cab, isolation mounts are typically used to support the rear of the cab on the chassis.
Prior truck cab suspension units have employed combinations of springs and shock absorbers to support the cab on the chassis while resisting the communication of shocks and vibrations from the chassis to the cab. In some of these devices, air springs have been employed, which are connected to the compressed air supply of the truck. The pressure inside these air springs is typically controlled by a height control valve, which maintains the extension of the spring constant regardless of the weight of the load carried in the cab.
However, air springs and shock absorbers are not alone sufficient in themselves to effectively support the rear of a tilt cab on a truck chassis. While there is considerable rigidity in the chassis and in the cab from their own structural components, the dynamic forces which the cab and chassis are subjected to during acceleration, braking and cornering make it desirable for the suspension units to have some rigidity of their own.